This invention relates to an adhesive resin composition based on an ethylene vinyl acetate polymer or an ethylene acrylate polymer. More specifically, it relates to an adhesive resin composition which is useful in peel and seal applications and demonstrates a whitening effect upon peeling. The whitening effect deters tampering by alerting a consumer that a sealed package has been opened.
Numerous adhesive resin compositions based on ethylene/vinyl acetate copolymers and ethylene acrylate copolymers are known in the art. For example, it is known to have an adhesive resin composition of (A) an ethylene/vinyl acetate copolymer having a vinyl acetate content of 5 to 40% to weight and (B) a tackifier, the ratio of A:B being 50-99:50-1. The tackifier may be rosin, rosin derivatives, aromatic hydrocarbon resins, aliphatic hydrocarbon resins, ester gum, cyclopentadiene resin, terpene resins and beta-pinene resin. The adhesive composition may further comprise a polyolefin resin, including a modified polyolefin having grafted thereto an unsaturated carboxylic acid or its anhydride. Such adhesive resin compositions can be applied to or extrusion-coated on low-density polyethylene, high-density polyethylene, ethylene/vinyl acetate copolymer, polyvinyl chloride, polyvinylidene chloride, polycarbonate, polystyrene, polypropylene, polybutylene, ABS, glass wood, cloth or other substrates.
Such adhesive compositions, however, do not prove to be satisfactory in every respect. For example, the packages or substrates which are adhered with such prior art adhesives are capable of being opened and subsequently resealed without evidence of tampering.
In the past, for example, in the packaging of certain types of food, candy bars, which require packaging sealed with a cold adhesive, a sealed package is capable of being opened and subsequently put back together without evidence of tampering. The present invention remedies this problem.
A number of tamper evident seals exist in the art. For example, U.S. Pat. No. 4,429,015 discloses a multiple laminate for identification cards. Attempts to delaminate the card results in fibers being torn from a uniaxially oriented polyethylene or polypropylene layer of the laminate.
U.S. Pat. No. 4,121,003 discloses switch-proof labels using a laminate having a pattern printed on the label inner surface and a coating of pressure-sensitive adhesive film on the inner surface. Attempts to transfer the label result in disruption of the printed pattern.
U.S. Pat. No. 4,931,327 discloses a tamper-evident film which may be used to provide a tamper-evident packaging seal. In forming a package seal, the tamper-evident film may be bound to either an identical or a different film with an adhesive, heat sealant, or heat seal. The adhesive, heat sealant, or heat seals have a seal internal cohesion and an affinity for bonded surfaces greater than the internal cohesive force of the tamper-evident film so that if a force is applied to separate the films, rupture is initiated by the cavitated skin, and tamper evidence occurs.
Such tamper-evident seals and laminates are expensive and difficult to produce. The prior art seals rely on the structure and configuration of the layers. Therefore, a simple, inexpensive adhesive seal having the desirable characteristics of the prior art and also having a tamper-evident property is much desired.